Determining magnetic field gradient is important for many things, including determining relative position and precise position location, and in identifying objects. Previously, magnetic field gradient has been determined by employing individual magnetic field sensors at widely spaced locations, and determining the magnetic fields at those locations.
A stress-driven magnetoelastic amorphous metal ribbon magnetometer which exhibits a minimum detectable field capability of 8.7 pT/.sqroot.(Hz) at 1.0 Hz has been described in Mermelstein, M.D., and Dandridge, A.: "Low frequency magnetic field detection with a magnetostrictive amorphous metal ribbon", Applied Physics Letters, 1987, Vol. 51, pp. 545-547. That device is constructed by interfacing a transversely field-annealed ribbon to a resonating piezoelectric plate with a viscous fluid. The piezoelectric plate is driven at the fundamental acoustic resonance corresponding to free end boundary conditions, thereby subjecting the mounted ribbon to an oscillating stress. The oscillating stress in the ribbon generates an oscillating magnetization whose amplitude is proportional to the signal field strength. The magnetization oscillations are detected with a centrally located pickup coil, and the signal field value is recovered with phase sensitive detection electronics.
Problems exist in the measuring of very small magnetic field gradients in geological surveying, medicine, and military applications. Widely separated magnetic field sensors encounter special problems with noise which requires complex systems for removal. Widely separated sensors are difficult to use and difficult to coordinate.